I'll take your word for it and assume you didn't cut the bike off....
If so, the fine sounds like complete bullshit!!
As others have said, if every idiot sat stationary at the merge point of a freeway onramp, waiting for a gap big enough to do 0 - 100 km/hr before being rear-ended, there would be traffic chaos and road rage aplenty!!
I don't know (or care for that matter....common sense rules IMO) if road rules have changed since I got my license but I was taught 1 in - 1 out when merging. You accelerate up to the speed of the lane you are merging into AS YOU REACH THE MERGE POINT, then with a little give and take, merge in and nobody needs to brake at all.
The idiots that speed up when they see a car approaching to merge, or in peak hour don't do the 1 in 1 out for queued traffic on onramps, are the people who need to be fined!! (and beaten severely )
Your fine would appear to support the latter (Oh no you don't!!) method of merging, which is farking ridiculous.
I would contest it, but obviously get legal advice before you do so and see what your chances of success are.
Traffic cops FTMFL.....fark I hate those wankers!!